Support efficient access to MapValue keys and values

[odenix]

Goals:

1. Efficiently iterate over a MapValue.
   Currently, MapValue can be iterated over by using one of the following methods:

   • MapValue.entrySet(), which allocates a Map.Entry object for every entry.
   • MapValue.getKeyValueArray(), which makes a defensive copy of the array.

   Iterating over MapValue.map() is no better than iterating over MapValue.entrySet().

2. Efficiently access a value whose entry index is known or can likely be guessed correctly.
   A common use case is constructing a Java object from a MapValue whose pack order is likely known.
   Currently, there is no way to benefit from this knowledge:

   Value get(Map<Value, Value> map, String key) {
     // this is O(n) and produces garbage, could be O(1) and no garbage
     return map.get(ValueFactory.newString(key));
   }
   record LargeRecord(String foo, int bar, ...);
   var map = mapValue.map();
   var large = new LargeRecord(
     get(map, "foo").asStringValue().asString(), 
     get(map, "bar").asIntegerValue().asInt(), 
     ...
   );
     
   

Proposal:

To achieve the goals described above, I propose to add the following methods to MapValue:

Value getKey(int entryIndex);
Value getValue(int entryIndex);

Results:

1. Iterating over MapValue produces no garbage:

   for (var i = 0; i < mapValue.size(); i++) {
     var key = mapValue.getKey(i);
     var value = mapValue.getValue(i);
   }
   

2. Accessing a value whose entry index is known or guessed correctly is O(1) instead of O(n);

   Value get(MapValue mapValue, String stringKey, int guessedIndex) {
     var key = mapValue.getKey(guessedIndex);
     return key.isStringValue() && stringKey.equals(key.asStringValue().asString()) ?
       // fast path
       mapValue.getValue(guessedIndex) : 
       // slow path
       mapValue.map().get(ValueFactory.newString(key));
   }
   var large = new LargeRecord(
     get(mapValue, "foo", 0).asStringValue().asString(),
     get(mapValue, "bar", 1).asIntegerValue().asInt(),
     ...
   );
   

Alternatives:

The first goal can be partially achieved by overriding method java.util.Map.forEach() in class ImmutableMapValueMap. This is PR #809 .
The second goal can sometimes be achieved by using Unpacker.unpackMapHeader() etc. instead of Unpacker.unpackValue().asMapValue().

If my proposal is approved, I'd be happy to send a PR.

[odenix]

Ended up writing my own MessagePack library.